The present invention relates to a developing roller for use in a developing apparatus such as a copying machine, a facsimile machine or a laser beam printer, and a method of producing the same. More particularly, the present invention relates to a developing roller for use in a non-magnetic one-component toner developing apparatus, and a method of producing the same.
Conventionally, a developing system which uses a magnetic two-component toner has prevailed in a developing apparatus such as a laser beam printer. However, in view of recently increasing concern about the global environmental preservation and the material saving, attention are being paid on a developing system using a non-magnetic one-component toner, which does not require recovery of the toner, generating no waste toner, and which uses out all the toner in the toner cartridge. Thus, efforts have been made to put this system into practical use.
The developing apparatus based on the non-magnetic one-component toner developing system basically has a photosensitive drum, electrostatic image-forming means for forming an electrostatic image on the photosensitive drum, a developing roller rotating in contact with the photosensitive drum, a toner-supplying member consisting of, for example, a urethane sponge supply roller, for supplying a non-magnetic one-component toner onto the developing roller, and a toner-regulating member consisting of, for example, a blade made of a urethane rubber or a urethane resin, for uniformly regulating the thickness of the toner supplied onto the developing roller. In general, while the electrostatic image-forming means forms an electrostatic image on the surface of the photosensitive drum based on the predetermined image information, the toner-supplying member supplies the toner onto the surface of the developing roller, and the toner-regulating member regulates the thickness of the toner supplied on the developing roller to form a uniform thin layer of the toner on the developing roller. The developing roller, on which the thin toner layer of a uniform thickness is formed, sequentially allows the toner to adhere to the electrostatic image formed on the photosensitive drum at the nip between it and the drum. In this way, the toner development is effected.
The developing roller, used in a developing apparatus of such a non-magnetic one-component toner developing system, allows the toner, which is positively or negatively charged by the frictional contact, to electrostatically adhere to its surface. The roller is constituted by an electrically conductive roller. Such an electrically conductive roller usually has a construction wherein an electrically conductive core (roller core) constituting the roller body has, on its cylindrical surface, an electrically conductive layer made of an electrically conductive material. Conventionally, as the electrically conductive material forming the electrically conductive layer, use is made of an electrically conductive rubber material containing a rubber material such as a silicone rubber, an acrylonitrile-butadiene rubber, a urethane rubber or a silicone-modified ethylene-propylene rubber, as a base, added to which is an electrically conductive material, such as carbon black or metallic powder, for imparting electrical conductivity thereto.
However, the silicone rubber and the silicone-modified ethylene-propylene rubber contains a low molecular weight siloxane therein, which migrates onto the surface, contaminating the surface of the photosensitive drum which contacts the developing roller. The acrylonitrile-butadiene rubber uses sulfur or a sulfur derivative as a vulcanizing agent, which contaminates the photosensitive drum surface. On the other hand, while the urethane rubber hardly contaminates the photosensitive drum, its volume resistivity value largely changes in accordance with the change of its environment (i.e., large in environmental dependency), lacking in practical utility.
In order to overcome the problems of the prior art rubber-based conductive layers, in particular, the contamination problem of the photosensitive drum surface, Japanese Patent No. 2504978 discloses a technique of applying a covering layer to a rubber-based conductive layer. The disclosed covering layer is made of a reaction product of a urethane resin with a fluorine-containing compound having a functional group, i.e., a fluorine-containing urethane resin. The covering layer can stop the migrating contaminants contained in the base rubber thereat, making it possible to prevent the contamination of the photosensitive drum surface by the migrating contaminants.
However, the developing roller provided with the fluorine-containing conductive layer gives rise to generation of so-called negative ghosts. For example, there occurs a problem that when an original having black regions in the background of characters is printed, the characters are slightly copied in the black regions in the printed sheet.
It is therefore a main object of the present invention to provide a developing roller which does not bring about the generation of negative ghosts, or which can suppress the generation of negative ghosts to a large extent.
A further object of the present invention is to provide a developing roller which does not contaminate the surface of the photosensitive drum, and yet does not bring about the generation of negative ghosts, and method of producing the same.
A still further object of the present invention is to provide a developing roller whose triboeletric series or electrostatic chargeability can be controlled to be suitable for either a positively charged toner or a negatively charged toner, and a method of producing the same.
According to a first aspect of the present invention, there is provided a developing roller comprising a roller core, an electrically conductive layer provided on a circumferential surface of the roller core, and a covering layer provided on a circumferential surface of the conductive layer, wherein the covering layer has a thickness of 30 xcexcm or less, but 4 xcexcm or more, and has at least its surface region formed of a porous body.
In the first aspect of the present invention, the pores in at least the surface region of the covering layer preferably have a size of 3 xcexcm or less, but 0.1 xcexcm or more, and the covering layer is preferably entirely formed of such a porous body.
In the first aspect of the present invention, the conductive layer is preferably formed of an electrically conductive rubber material.
In the first aspect of the present invention, the covering layer usually comprises a reaction product of a polyol with an isocyanate compound. Alternatively, the covering layer may comprise a reaction product of a polyol, an isocyanate compound, and a reactive silicone oil having active hydrogen. In these cases, the polyol is particularly preferably a fluorine-containing polyol.
According to a second aspect of the invention, there is provided a developing roller comprising a roller core, an electrically conductive layer provided on a circumferential surface of the roller core, and a covering layer provided on a circumferential surface of the conductive layer, wherein the covering layer is formed by subjecting a reaction mixture containing a polyol, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil to a reaction condition for reacting the polyol and the isocyanate compound.
To produce this developing roller, the present invention provides a method of producing a developing roller comprising the steps of providing a roller core having an electrically conductive layer provided on a circumferential surface thereof, applying a reaction mixture containing a polyol, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil to a surface of the conductive layer, and subjecting the reaction mixture to a reaction condition for reacting the polyol and the isocyanate compound.
Further, according to the present invention, there is provided a developing roller comprising a roller core, an electrically conductive layer provided on a circumferential surface of the roller core, and a covering layer provided on a circumferential surface of the conductive layer, wherein the covering layer is formed by subjecting a reaction mixture containing a polyol, a reactive silicone oil having active hydrogen, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil to a reaction condition for reacting the polyol and the reactive silicon oil with the isocyanate compound.
To produce this developing roller, the present invention provides a method of producing a developing roller comprising the steps of providing a roller core having an electrically conductive layer provided on a circumferential surface thereof, applying a reaction mixture containing a polyol, a reactive silicone oil having active hydrogen, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil to a surface of the conductive layer, and subjecting the reaction mixture to a reaction condition for reacting the polyol and the reactive silicone oil with the isocyanate compound.
In the present invention, the polyol is preferably a fluorine-containing polyol, and is particularly preferably a copolymeric polyol containing ethylene tetrafluoride monomer units as the main component.
In the present invention, the reactive silicone oil is preferably represented by the following formula (1) or (2): 
In the formulas (1) and (2), each R represents xe2x80x94C3H6OC2H4OH or xe2x80x94C3H6OCH2xe2x80x94C(CH2OH)2C2H5, and n is an integer of about 20 or less.
In the present invention, the triboeletric series or electrostatic chargeability of the covering layer can be controlled by changing the mixing ratio between the fluorine-containing polyol and the reactive silicone oil.
Further, in the present invention, the conductive layer is preferably formed of at least one rubber selected from silicone rubber, an acrylonitrile-butadiene rubber, a silicone-modified ethylene-propylene rubber, and a urethane rubber, each imparted with electrical conductivity.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.